The present invention relates to methods for forming semiconductor devices. In particular, the present invention relates to methods for forming dielectric layers on semiconductor devices.
Dielectric layers on semiconductor devices are used to isolate or protect other layers. Currently silicon dioxide, silicon nitride and silicon oxynitride are the most commonly used dielectric layers.
One type of dielectric layer is a thermally-grown silicon dioxide. Thermally-grown oxides are formed by reacting silicon on the substrate with oxygen at high temperatures. Field and gate oxide layers are examples of thermally-grown oxides.
Another type of dielectric layer is a deposited dielectric layer. Deposited dielectric layers include interlevel dielectric layers such as boro-phosphorous silicate glass (BPSG), inter-metallic dielectric layers and passivation layers. Typically, deposited dielectrics are formed with a chemical vapor deposition (CVD) system. Common CVD systems include plasma enhanced chemical vapor deposition (PECVD) and low pressure chemical vapor deposition (LPCVD). In chemical vapor deposition, reactants in a gas or vapor react to form a dielectric material which is deposited on the substrate. To form a deposited silicon dioxide layer, typically a mixture of nitrous oxide (N20), silane (SiH4) and tetra-ethyl-ortho-silicate (TEOS) are used. To form a nitride layer, such as Si3N4, the reactant substances include silane and ammonia NH3. To form a combined oxynitride, a mixture of silane, nitrous oxide and ammonia is used.
A concern with metal-oxide-silicon field-effect-transistors (MOSFETs) is the need to maintain stable and reliable threshold and breakdown voltage values. It is desired to improve the processing steps used to form the dielectric layers to help obtain stable and reliable threshold and breakdown voltage values for MOSFETs.
The CVD deposition of dielectric layers typically uses reactant substances which include hydrogen. These reactant substances include silane, ammonia and TEOS. Such CVD depositions produce hydrogen gas as a by-product. Especially when a plasma is used, the hydrogen gas can break down into free radicals which do not easily recombine. These hydrogen free radicals become a part of the deposited dielectric layer as it forms.
It has been found that hydrogen can be between five to fifteen percent by weight of a deposited dielectric layer. Because the hydrogen free radicals are charged, hydrogen free radicals entrapped in the dielectric layer can adversely affect MOSFET performance. The entrapped charged particles can vary the threshold voltage and breakdown voltage for a MOSFET. Such changes may not be immediately apparent and may occur after the integrated circuit (IC) is sent to a customer. Additionally, a signal pattern can cause the entrapped hydrogen free radicals to migrate and adversely affect the performance.
The present invention involves supplying deuterium into the CVD chamber. Deuterium competes with the more common isotope of hydrogen to enter the dielectric layer. Additionally, deuterium is more stable than the common isotope of hydrogen and is thus less likely to form free radicals which can adversely affect the MOSFET performance.
Deuterium has been used in the past in the forming of polysilicon layers. Deuterium affects the micro-crystalline structure of the polysilicon layer as it forms. See, for example, T. Shiraiwa et al.; xe2x80x9cCharacterization of Chemical-Vapor-Deposited Amorphous-Silicon Filmsxe2x80x9d; Jpn. J. Appl. Phys. 32; Jan. 15, 1993; Pt. 2, No.1A/B; pp. L20-23; and E. Srinivasan and G. N. Parsons; xe2x80x9cHydrogen elimination and phase transitions in pulsedgas plasma deposition of amorphous and microcrystalline siliconxe2x80x9d; J. Appl. Phys.; Vol. 81, No. 6; Mar. 15, 1997; pp. 2847-2855. In its prior use, deuterium was not used to form a dielectric layer. Additionally, dielectric layers do not require a specific micro-crystalline structure. Furthermore, in the prior art, deuterium was not used to compete with a hydrogen by-product during a layer deposition.
In one embodiment of the present invention, deuterium is added in the deposition of the dielectric layer when the deposition is caused by reactive substances which produce hydrogen by-products.
Another embodiment of the present invention is a chemical vapor deposition system arranged with a deuterium supply and reactant substance supplies, where the reactant substances form a hydrogen by-product.
Another embodiment of the present invention comprises a semiconductor device having a dielectric layer including at least some deuterium. Having a dielectric layer including at least some deuterium has advantages over dielectric layers that include hydrogen by-products which are not deuterium.